The mobile or stator turbine blades, which are subject to high thermal stresses, include resources for cooling by the circulation of a heat-bearing fluid, generally air in the case of a gas-turbine engine, inside cavities created inside the vane.
In addition, the cooled turbine blades are now generally manufactured by the lost-wax moulding process. This technique has several stages. A first stage consists of creating a model of the part that one wishes to mould, in wax or another equivalent material. In a second stage, one then surrounds the model in a shell mould made from a ceramic material. The latter is manufactured by successive soakings of the model in slurries containing a ceramic material, alternating with stucco operations, between the soakings, on the layer formed. When it has been formed, the mould is dried and the wax that it contains is removed by a first firing at a suitable temperature, and then the mould is further fired at a high temperature in order to provide it with the strength necessary for the pouring process. This then leaves the hollow image of the model, into which the molten metal is poured. After cooling, the mould is broken to release the part. The latter is then subjected to a simple finishing process.
When the vane includes cavities for the circulation of a cooling fluid, it is necessary to incorporate one or more cores into the model, before the manufacture of the shell mould. This phase of the process includes firstly the separate manufacture of the core or cores by the moulding of a ceramic material consolidated by a binder, followed by their assembly where necessary, and the positioning of the core or cores inside a wax mould. The said model is thus moulded by the injection of wax into the wax mould. The wax fills the spaces created between the core or the assembly of core elements and the volume of the wax mould. The model obtained in wax forms the replica of the part to be manufactured, with the difference that the internal cavities are occupied by the core or cores.
During the alloy pouring stage, the molten metal flows between the walls of the shell mould and those of the core. After appropriate cooling of the alloy, the shell mould is firstly removed and then the elements constituting the core, in order to produce the cavities. The part finally undergoes a simple finishing process.
The technique presented above allows the creation of parts with complex internal geometries. The patent application in the name of the applicant, FR 0,309,535, filed on 1 Aug. 2003, corresponding to U.S. Pat. No. 7,033,136, describes a turbine blade with several cooling circuits, a first along a central cavity and cavities on the upper and lower surfaces of the vane, with a second that is independent of the first circuit, along the leading edge of the blade, and a third which is independent of the first two, along the trailing edge of the blade.
A high-pressure turbine blade 10 for a gas-turbine engine is shown in FIGS. 1 and 2. The blade includes an aerodynamic surface 12 called a vane, which extends in a radial direction between a blade root 14 and a blade tip 16. The root is shaped so as to allow mounting of the blade on a rotor disk. Here, the tip of the blade has a part said to be in the shape of a bathtub 28. This is composed of a bottom 26 transverse to the vane and a wall forming its edge as an extension to the wall of the vane 12.
FIG. 2 shows the vane in section, slightly enlarged, in the plane II-II of FIG. 1. Here the vane 12 includes a variety of cavities. A first cavity is central 34, and extends from the root up to the tip of the vane. Three cavities 30 are located along the wall of the upper blade surface between the central cavity 34 and the upper blade wall, and along the latter. The cavities 30 are narrow and positioned behind each other along this wall. Two cavities 32 are located between the central cavity and the lower blade surface, and along the latter. These also are narrow. One cavity 50 is located in the part of the blade close to the trailing edge. One cavity 60 is located in the part of the blade close to the leading edge.
FIG. 3 shows a view of the blade in section in the plane III-III of FIG. 2. The cooling air first circulates along the upper-surface and lower-surface cavities and then is routed to the central cavity, to be extracted via openings created appropriately. One or two different circuits feed air into the two cavities labelled 50 and 60.
For the manufacture by moulding of a part with such a structure, cores are created corresponding to the different cavities, in the form of a first core 1 whose shape corresponds to that of the first cavity, a second core 2 whose shape corresponds to the three cavities 30 of the upper blade surface and a third core 3 whose forms corresponds to the two lower-surface cavities 32. Cores are also created for cavities 50 and 60. In the remainder of the presentation however, we will consider only the first three cavities with their associated cores to the extent that they form a single cooling circuit.
FIG. 4 shows a partial view of the tip of the part in its state following the cooling of the metal. It is shown in section in a radial direction, after the shell mould has been removed. The cavities created inside the wall 13 of the blade are occupied by the assembled cores 1, 2 and 3, which now have to be removed.
It can be seen that the three cores have been created so that they can constitute a rigid assembly during the moulding operations. In fact it is important that the different elements should remain in position, and not be disturbed during the pouring of the molten metal in particular. As can be seen in FIG. 4, the three cores fit into the zone at the tip of the blade, at the position where a passage has to be created between the three cavities. They form a compact rigid assembly at this position. The central core 1 includes a part forming the body 1C corresponding to the central cavity 34, a slimmed-down part 1D and a wider head part 1E. The height of the slimmed-down part 1D corresponds more or less to the height of the passage created between the cavities. Core 2 includes a part forming the body 2C, here in three parts, corresponding to the three upper blade surface cavities 30a, 30b, 30c. These three parts 2C are joined at the tip of the blade to an element 2D inserted between parts 1C and 1E of the core 1, resting against part 1D. In like manner, core 3 includes a part forming a body 3C in two elements corresponding to the two lower-surface cavities 32a and 32b. These two elements are connected by an element 3D at the tip of the blade, which is inserted between parts 1C and 1E of core 1, resting against part 1D. This assembly allows the manufacture of the blade by pouring of the molten metal between the shell mould and the cores, thus creating the wanted cavities and passages.
However, as can be seen in FIG. 4, in practice an interstitial space remains between the parts of the cores thus assembled. The molten metal occupies the space left by these parts. This means that after the removal of the cores, there remain flashings B1 to B4. These metal flashings must be removed since they form a location at which cracks appear during operation of the machine on which the part is mounted. They also result in pressure drops in the flow of the cooling fluid inside the blade.
These are therefore removed by chemical or mechanical means. These operations are lengthy and difficult, and their cost is not negligible. Moreover, it is necessary to create a relatively large opening in the transverse wall forming the bottom of the bathtub, through which one introduces the tool used for removal of the flashings. An edge is created along this opening, and this serves as a base for a close-off plate forming the bottom of the bathtub. We have already seen that because of the size of this opening, the plate is prone to collapsing, leading to significant operational disruptions.